John is checking out the Amazon.com website and he wants to buy a book. At the checkout page he enters his credit card information and hits submit.
When he bought his coffee at Starbucks, the card reader read the credit card information, converting it in such a way that the information could be sent to the banking network.
When credit card information is accepted over the Internet, something equivalent of the “card reader” is needed. This will convert the credit card number and other information to a data format that can be sent to the banking network. This is what Gateways do. Popular Gateways are Authorize.net, Braintree and Samurai.
In short, Gateways: